The National Electric Code (“NEC”) dictates that, for safety reasons, before a coaxial cable enters a residence, there must be a cable shield grounding point at the point of entry. The coaxial shield should make contact with an earth-grounded wire of a size no smaller than #8 for Aluminum or no smaller than #10 for solid copper. Grounding serves the additional purpose of reducing equipment damage from lightening.
The present-day grounding approach is to use a device called a “grounding block.” FIGS. 1A-1C show top views of exemplary grounding blocks, showing single 10, dual 10′, and quad 10″ versions, respectively, including F-connector terminals. The grounding blocks may also serve as a connection point for one or more grounding wire(s) for other devices/components, such as a dish antenna or an off-the-air antenna.
With reference to FIG. 1A, an F-connector grounding block 10 may be used to ground a coaxial cable, e.g., from an antenna mounted to a residence (not shown). A first male F-connector may be mated or otherwise provided on an end of a first length of coaxial cable (e.g., that extends from the antenna), and the first connector may be threaded into an F-connector input 12 on the grounding block 10. A second male F-connector is mated with a second coaxial cable (e.g., that extends into the residence), and the second connector may be threaded into an F-connector output 14 of the grounding block 10 opposite the F-connector input 12 (e.g., if the grounding block includes multiple connections). For example, a coaxial cable may be cut, and connectors mated to the cut ends before connecting the ends to the F-connectors 12, 14 of the grounding block 10.
The grounding block 10 may include one or more holes to receive grounding wires (not shown), e.g., #8 Aluminum or #10 solid copper grounding wires, and one or more screws 16 that may be tightened to secure the grounding wires in respective holes. The grounding wire(s) may be extended to nearby earth ground point(s), e.g., per codes by the NEC.
Other known methods for grounding coaxial cables involves stripping or otherwise removing the cable jacket to expose the underlying shield and cutting into the cable's braided shield and dielectric layers. Stripping or removing the jacket may be time-consuming and cutting into the braided shield and dielectric layers may alter the characteristic impedance of the coaxial cable. Such impedance changes may be acceptable for low frequency signals, e.g., in the Kilohertz (KHz) range, such as those encountered in audio applications, but are generally unacceptable for high frequency signals, e.g., radiofrequency (“RF”) signals in the Megahertz (MHz) and Gigahertz (GHz) ranges.
Two of the primary problems associated with grounding blocks, such as those shown in FIGS. 1A-1C are the extra time required to make the F-connector/cable assemblies, and the cost of the F-connectors required for each end of a spliced cable. For example, with a four-cable distribution system using a quad grounding block, such as that shown in FIG. 1C, a total of eight (8) F-connector/cable assemblies must be fabricated on site. This may be further inconvenient given that the connectors and connections may be made at inconvenient locations, e.g., near the roofline of the residence.
Another more subtle but performance-impacting issue is the potential degradation of Return Loss (“RL”) from a resultant connection (also known as more signal reflection). RL is a measure of how closely the characteristic impedance of a connection matches. A mismatch increases RL and degradation of signal power transfer (more reflection). The higher the signal frequency, the greater is the potential for degradation. Everything else being equal, more connections in a given system results generally in more RL degradation.
Another potential for RL degradation with the conventional grounding blocks is moisture. Grounding blocks, such as those shown in FIGS. 1A-1C, are not easily isolated from moisture, e.g., requiring that they be enclosed in a sealed container or wrapped with a weather boot. Moisture in a connection may also change the system characteristic impedance.
The coaxial cable's characteristic impedance “Z0” (measured in ohms) is determined by the following cable design formula:Z0=138.2/√Er log(D/ad)
where “Er” is the dielectric constant of the cable core, “D” is the dielectric diameter, “d” is the conductor diameter, and “a” is the conductor strand factor.
As the signal frequencies extend higher, parasitic parameters may affect the basic formula more to make the Z0 deviate from its intended value, thus causing RL degradation.
Accordingly, apparatus and methods for grounding coaxial cables or other electrical cables would be useful.